The peak amplitude of the fundamental torsional
mode, T(0,1), ultrasonic guided wave, excited and
sensed by commercially available equipment using
permanently mounted magnetostrictive transducers,
was used to monitor the growth of electrochemically
induced, external discontinuities in a
steel pipeline (216 mm outside diameter). The
pipeline was more than 30.5 m long, had multiple
welds and two 90° elbows. Over the course of
eight months, two discontinuities, hidden from
ultrasonic testing (UT) technicians, grew systematically
in both area and depth, providing the opportunity
to experimentally evaluate the effects of
complex geometry on signal characteristics.
Corrosion around a weld was also studied. The
obtained results were compared with a surface
discontinuity that was unaffected by intermediate
welds located between the transducer and the
discontinuity. It was found that the effects of
multiple welds and an elbow decreased the sensitivity
for discontinuity growth monitoring by a
factor of less than two. This was established by
examining the scaling of discontinuity signal peak
heights against a weld peak instead of using a
conventional distance-amplitude-correction (DAC)
curve to account for material attenuation, by
comparing the signals from a discontinuity viewed
along the forward and the reverse direction, and
by allowing the discontinuities to change in all
three dimensions in the course of this monitoring
effort. This controlled study for discontinuity
growth monitoring can be useful to guide efforts
on discontinuity growth monitoring of complex
pipelines in the field.